At any age, your risk of suffering any smoking related illness begins to drop steadily from the moment you quit smoking, whereas continuing to smoke means odds of illness or early death continue to increase. In one study in the U.K., cessation at age 60, 50, 40, or 30 years gained, respectively, about 3, 6, 9, or 10 years of life expectancy. In another large U.S. study, men who quit smoking at age 65 gained 1.4 to 2.0 years of life and women gained 2.7 to 3.7 years of life. When a person quits smoking, circulation improves immediately, and the lungs begin to repair damage. In one year after quitting, the added risk of heart disease is cut almost in half, and risk of stroke, lung disease, and cancer diminish.
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Taylor DH, Hasselblad V, Henley J, Thun MD, & Sloan FA. Benefits of Smoking Cessation for Longevity. American Journal of Public Health. 2002; 92:990-6.